An electrolyzer and a fuel cell can be cited as examples of an electrochemical device where a polymer solid electrolyte is used as an ion conductor in place of a liquid electrolyte. Polymer membranes used for these must be sufficiently stable chemically, thermally, electrochemically and mechanically and must have a high proton conductivity as a cation exchange membrane. Therefore, a perfluorocarbon sulfonic acid membrane, of which a typical example is “Nafion (registered trade mark) made by DuPont Inc., USA” has been utilized as a polymer membrane which can be utilized for a long period of time. In the case where a Nafion membrane is operated under the condition where the temperature exceeds 100° C., however, the water content of the membrane dramatically decreases and the membrane is significantly softened. Therefore, in a promising fuel cell where methanol is used as the fuel reduction of performance occurs due to the crossover of methanol through the membrane and the fuel cell cannot exercise a sufficient performance. In addition, also concerning a fuel cell that is operated at a temperature close to 80° C. using hydrogen as the fuel which is presently examined as a mainstream technology, an excessively high cost of the membrane is pointed out as an obstacle impeding the establishment of a fuel cell technology.
In order to overcome such defects, a variety of polymer electrolyte membranes where a sulfonic group has been introduced into a polymer that contains non-fluorine-based aromatic ring have been examined. A polymer skeleton of aromatic polyether-based compounds, such as polyarylene ether ketones, polyarylene ether sulfones is considered to be a promising structure, taking heat resistance and chemical stability into consideration and sulfonated polyarylene ether sulfones (see, for example, Journal of Membrane Science (Netherlands) 1993, vol. 83, pp. 211-220), sulfonated polyether ether ketones (see, for example, Japanese Laid-Open Patent Publication No. 6-93114 (pp. 15-17)), sulfonated polystyrenes and the like have been reported.
From among these, a sulfonated polyarylene ether that includes 4,4′-biphenol as a monomer is also reported (see, for example, WO 00/24796) which has a problem where the polymer swells under high temperature and high humidity and, in particular, this tendency becomes significant in compositions where the ratio of sulfonation becomes high. In addition, sulfonic groups that have been introduced to an aromatic ring as a result of a sulfonation reaction of these polymers in general tend to be eliminated due to heat and according to a method for obtaining a compound that has been improved in this issue, a monomer where a sulfonic group has been introduced to an electron withdrawing aromatic ring is polymerized and, thus, a sulfonated polyaryl ether sulfone-based compound of which the thermal stability is high has been reported (see, for example, US Patent No. 2002/0091225 (pp. 1-2)). In this case, a problem arises where polymerization for a long period of time is required in order to obtain a polymer due to a low reactivity of the monomer (see, for example, ACS Polymer Preprints (USA) 2000, vol. 41(2), pp. 1388-1389). This reference describes usage of 4,4′-biphenol as another monomer for copolymerization, wherein a problem also arises where the polymer swells under high temperature and high humidity and, in particular, this tendency becomes significant in compositions where the ratio of sulfonation becomes high.